This invention relates to the preparation of fiber-reinforced thermosettable resin articles. In a specific embodiment, the invention relates to a process and apparatus for reducing resin scrap in the impregnation of a glass substrate with a solventless thermosettable resin system.
The manufacture of the cured thermosettable resin base of an electronic circuit board begins with the impregnation of a fibrous glass substrate with a liquid thermosettable resin system. The resin-impregnated substrate is then partially cured to form a "prepreg." A set of layered prepregs is heated under pressure to fully cure the resin and to form a hard laminate, which serves as the base for electric circuitry.
Although there exist thermosettable resins, such as low molecular weight epoxy resins, which are liquid at room temperature, current circuit board requirements make it necessary to use high-performance resins systems which are solids or viscous liquids at room temperature and to apply the resins to the substrate in melt or solution form. Processing thermosettable resins in the melt, however, is difficult because the high temperatures necessary to melt the resin cause the resin to cure prematurely, resulting in poor "wet-out," or saturation of the substrate by the resin.
Current commercial processes for preparing prepregs apply the resin to the substrate using an organic solution of the resin. Solution processes must include a step, usually carried out in conjunction with partial curing of the resin, in which the solvent is removed from the prepreg by heating the solvent to its volatilization temperature. Such a process has a number of disadvantages: First, it requires the disposal or discharge of the organic volatiles. Second, volatilization of the solvent from the uncured resin can result in the presence of voids and irregularities in the prepreg and in the cured laminate. Furthermore, a considerable amount of time is required for the solvent removal step.
A process has been proposed for application of a solventless thermosettable resin to a glass web which involves depositing the solventless resin onto a rotating roller and passing the glass web in countercurrent contact with the deposited resin so as to transfer the resin to the web, which is then passed to a heating zone for partial curing of the resin. It has been found that this resin application process can produce an undesirably high amount of scrap resin as a result of retention of a certain mount of resin on the rotating roller.
It is therefore an object of the invention to provide an improved process for impregnation of a fibrous substrate with a solventless thermosettable resin system which reduces the quantity of scrap resin produced in the process.